Solar panels that harness solar energy and convert it to electrical energy are well known. Typically, a solar panel has four basic parts: the photovoltaic (PV) cell, charge controller, battery, and inverter. Of the four parts, the PV cell is a p-n junction diode capable of generating electricity in the presence of sunlight, and is often made of crystalline silicon (e.g., poly-crystalline silicon) that is doped with elements from either group 13 (group III) or group 15 (group V) on the periodic table. When these dopant atoms are added to the silicon, they take the place of silicon atoms in the crystalline lattice and bond with the neighboring silicon atoms in almost the same way as the silicon atom that was originally there. However, because these dopants do not have the same number of valence electrons as silicon atoms, extra electrons or “holes” become present in the crystal lattice. Upon absorbing a photon that carries an energy that is at least the same as the band gap energy of the silicon, the electrons become free. The electrons and holes freely move around within the solid silicon material, making silicon conductive. The closer the absorption event is to the p-n junction, the greater the mobility of the electron-hole pair.
When a photon that has less energy than silicon's band gap energy strikes the crystalline structure, the electrons and holes are not mobilized. Instead of the photon's energy becoming absorbed by the electrons and holes, the difference between the amount of energy carried by the photon and the band gap energy is converted to heat.
While the idea of converting solar energy to electrical power has much appeal, the conventional solar panels have limited usage because their efficiencies are generally only in the range of 15%. This low efficiency is due in part to the planar configuration of current PV cells, as well as the relatively large distances between the electrodes and the P-N junction. Low efficiency means that larger and heavier arrays are needed to obtain a certain amount of electricity, raising the cost of a solar panel and limiting its use to large-scale structures.
The most common material for solar cells is silicon. Crystalline silicon comes in three categories: single-crystal silicon, polycrystalline silicon, and ribbon silicon. Solar cells made with single or monocrystalline wafers have the highest efficiency of the three, at about 20%. Unfortunately, single crystal cells are expensive, and round so they do not completely tile a module. Polycrystalline silicon is made from cast ingots. They are made by filling a large crucible with molten silicon and carefully cooling and solidifying them. The polycrystalline silicon is less expensive than single crystal, but is only about 10-14% efficient depending on the process conditions. Ribbon silicon is the last major category of PV grade silicon. It is formed by drawing flat, thin films from molten silicon, and has a polycrystalline structure. Silicon ribbon's efficiency range of 11-13% is also lower than monocrystalline silicon. Most of these technologies are based on wafers about 300 μm thick. The PV cells are fabricated then soldered together to form a module.
Another technology under development is multijunction solar cells, which is expected to deliver less than 18.5% efficiency in actual use. The process and materials to produce multijunction cells are enormously expensive. Those cells require multiple gallium/indium/arsenide layers. The best to date is believed to be a sextuple-junction cell.
A promising enabler of PV cells and other technology is nanotechnology. However, one problem with implementing nanotechnology is that the minute conductors may not be able to withstand subsequent processing conditions. For example, the metal forming the conductors may melt during high temperature processes such as chemical vapor deposition (CVD).
Thus, as alluded to, the technology available to create PV cells and other electronic structures is limited to some extent by the high cost of fabrication as well as processing limitations.
Therefore, it would be desirable to enable creation of nanostructures having high aspect ratios and yet are able to survive processing such as CVD.
It would also be desirable to enable fabrication of a solar cell that has a higher than average efficiency, and in some embodiments, higher than about 20%.